editing forum posts: why doesn't delete work?

If you start a thread and nobody replies you can delete it, I've never tryed to delete one of my threads after it has gotten a reply so I don't know if that works. Also I can delete this post also so apparently that works to. I'm thinking that if you start a thread and it gets replied to, then you can't delete it. I've never tryed but that would make sense to have it that way.